1. Field of the Invention
This invention relates to improvements in pulse density modulator circuits, and more particularly to improvements in pulse density modulator circuits of the type which receive sequentially applied input parallel digital words or signals.
2. Relevant Background
Many applications exist in which it is desirable to convert a series of parallel digital words into a pulse density signal. For example, in many telecommunications applications, voice or other sounds are digitized and transmitted as parallel digital words. Upon reception, the digital words are converted into a form that can be converted into audio, reproducing the original voice or sounds. One way by which this has been done has been by converting the received digital words into a pulse density modulated signal, then passing the pulse density modulated signal through a low pass filter to produce an analog signal.
Typically, the pulse density modulation of the input digital words involves framing the words, that is, treating each, word individually to produce a set output stream of pulses of length corresponding to the number of bit values that may be represented by the input word. For example, if an n-bit input word is modulated, the pulse density modulation signal requires a frame of 10 pulses long to represent the word. However, if the pattern is changed in the middle of the frame discontinuities at the output may occur. On the other hand, if the pattern is only allowed to be changed at the end of a frame, the delay is quantized to the frame length, which is 2.sup.n clocks.
Thus, what is needed is a method and apparatus for providing a pulse density modulated signal to represent an input sequence of parallel words that can be changed without resulting in output discontinuities, and which does not require data framing.